Controlling the deposition of nano-dimensioned solids at the nanometer scale has the potential to revolutionize technology through development of materials and devices with control of mechanical, optical, electronic and structural properties. Moreover, recent research has led to a host of new fundamental scientific insights, including controlled nanoscale synthesis and processing of both organic (soft) and inorganic (hard) material and the development of nano-scale precursors for these macroscopic materials and devices. A challenge, therefore, is to develop an approach that can combine a variety of organic and inorganic building blocks, provide down to nanometer-scale structural control and simultaneously lead to macroscopic devices or materials in a practical and cost-effective way. Moreover, the approach must be flexible so that it can be readily extended to a variety of materials or properties without substantial revision of the entire process. These are demanding goals that require novel approaches and development of basic science.
Conventional metal deposition methods such as sputtering or evaporation have poor selectivity, required elevated temperature and need special vacuum systems. Methods such as dip-coating using colloidal suspensions take long time and are difficult to apply on thin and bendable substrates. Photolithography provides a means of generating structure, generally planar in nature, with a spatial resolution on the nanometer to micron size scale, but this technique is limited to a small set of materials.
Chemical synthesis, for example synthesizing carbon and other nanotubes, can provide molecular resolution, but is limited in its ability to independently control mechanical, structural, electronic and optical properties of a material.
One technique of recent interest involves the selective deposition of nano- or micro-dimensioned particles by self-assembly. Self-assembly is a term used to define the spontaneous association of entities into structural aggregates. In particular, molecular self-assembly provides the basis for a successful strategy for generating large, structured molecular aggregates, by the spontaneous association of molecules. See, for example, Whitesides, et al., in “Noncovalent Synthesis: Using Physical-organic Chemistry to Make Aggregates”, Accts. Chem. Res., 28, 37-44 (1995); Whitesides, G. M., “Self-Assembling Materials”, Scientific American, 273, 146-149 (1995); Philip, et al., Angew. Chem., Int. Ed. Engl., 35, 1155-1196 (1996).
Self-assembly of molecules can be made to occur spontaneously at liquid/gas, liquid/liquid, or solid/liquid interfaces to form self-assembled monolayers of the molecules when the molecules have a shape that facilitates ordered stacking in the plane of the interface and each includes a chemical functionality that adheres to the surface or in another way promotes arrangement of the molecules with the functionality positioned adjacent the surface. U.S. Pat. No. 5,512 131, and U.S. patent application Ser. Nos. 08/695,537, 08/616,929, 08/676,951, and 08/677,309, and International Patent Publication No. WO 96/29629, all commonly-owned, describe a variety of techniques for arranging patterns of self-assembled monolayers at surfaces for a variety of purposes.
Much of the literature in this area describes the self-assembly of forming extended colloidal structures, but several techniques are described for forming such nano- and microscale patterning, including tethering colloidal gold nanoparticles to surfaces with thiol groups (Mirkin, et al., A DNA-Based Method for Rationally Assembling Nanoparticles Into Macroscopic Materials, Nature, 382, (Aug. 15, 1996)).
The concept of using capillary action to deposit colloid or nano-materials has been described as useful in providing patterned self-assembled arrays. Yamaki, et al., in “Size Dependent Separation of Colloidal Particles in Two-Dimensional Convective Self-Assembly” Langmuir, 11, 2975-2978 (1995), relies on lateral capillary force and convective flow to provide “convective self-assembly” of colloidal particles ranging in size from 12 nm to 144 nm in diameter in a wetting liquid film on a mercury surface. Cralchevski, et al., in “Capillary Forces Between Colloidal Particles” Langmuir, 10, 23-36 (1994), describe capillary interactions occurring between particles protruding from a liquid film due to the capillary rise of liquid along the surface of each particle.
Shi-Kai Wu, et al., “Self Assembly of Polystyrene Microspheres Within Spatially Confined Rectangular Microgrooves,” J. Matl. Sci., 43 (19), 6453-6458 (2008) describes the use of capillary action to self-assemble 262 to 1000 nm polystyrene spheres onto patterned silicon wafers with one-dimensional microgrooves of different widths (0.76 -6 microns). Processing variables including evaporation temperature, particle size, groove width, and groove height were examined to explain the results.
O-Ok Park, et al., “Method for Manufacturing Colloidal Crystals Via Confined Convective Assembly,” U.S. Pat. No. 7,520,933, issued Apr. 21, 2009, discloses methods of manufacturing colloidal crystals using a confined convective assembly, comprising infusing colloidal suspension between two substrates and self-assembling the particles by capillary action. Substrates may include glass, inorganic and organic polymers; particles may include high molecular weight polymers, inorganic polymers, metals, and metal oxides. Solvents useful for the convective transfer include water and alcohol.
Peng Jiang, et al., “Polymers Having Ordered Monodisperse Pores and Their Corresponding Ordered, Monodisperse Colloids,” U.S. Pat. No. 6,929,764 (issued Aug. 16, 2005) describes the deposition of nano-silica “according to an appropriate technique, such as . . . convective self-assembly method.”
U.S. Pat. No. 5,45,291 (Smith) describes assembly of solid microstructures in an ordered manner onto a substrate through fluid transfer. The microstructures are shaped blocks that, when transferred in a fluid slurry poured onto the top surface of a substrate having recessed regions that match the shapes of the blocks, insert into the recessed regions via gravity. U.S. Pat. No. 5,355,577 (Cohn) describes a method of assembling discrete microelectronic or micro-mechanical devices by positioning the devices on a template, vibrating the template and causing the devices to move into apertures. The shape of each aperture determines the number, orientation, and type of device that it traps.
Self-assembly on patterned surfaces is particularly useful as a way of making nano- and microscale devices, for example electronic and electrochemical systems, sensors, photonic devices, biosensors and devices, information storage medium, display devices and optical devices, and medical (e.g., drug release) devices.
However, when attempting to apply convective self-assembly, several problems become evident. These particular problems include difficulties in depositing colloidal particles into high aspect ratio trenches or wells.
The main problem in hydrophobic structures with high aspect ratio is that water cannot penetrate and touch the bottom surface, so it is impossible to use liquid assembly techniques as dip-coating or convective assembly. Conventional plastic substrates show water contact angles around 100° and they are usually reduced applying O2 plasma or UV radiation to make the surface hydrophilic (contact angle below 20°). This problem is exacerbated in high aspect ratio nanostructures showed super-hydrophobicity (130°) before applying O2 plasma and a high contact angle (90°) after the plasma was applied. Also, O2 plasma is known to destroy or erode plastic patterns.
Another problem is that plastic substrates are usually thin and easy to bend and it is difficult to make conformal assembly at large areas.
Still another problem is that the time necessary for particles to move from, typically, aqueous dispersions into high aspect ratio features (e.g., vias and trenches) tends to be long. All of these problems become increasingly acute as the dimensions of the vias and trenches shrink, and are especially problematic for nano-dimensioned features.
What is needed is a versatile technique for facilitating convective self-assembly that accommodates a wide range of nano- or microparticles, works quickly over large areas, when the particles (or other nano- or micro-building blocks) have to be assembled into deep trenches or vias, whether the surface is hydrophobic or hydrophilic, without surface treatment.